Mount shared folder (vbox) as another user

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I'm trying to mount my vbox shared folder every time my ubuntu starts.
So, I added an entry on /etc/init with this:
description "mount vboxsf Desktop"
start on startup
task
exec mount -t vboxsf Desktop /var/www/shared
Seems to work, except by the fact that all the files are owned by "root", and I don't have permission to write on the folder (neither chmod nor chown seems to be working).
So, how can I make all the files under this shared folder to be owned by www-data user/group?
Thanks
ps.: The main reason for me to have an automatic shared folder, is so I can create/edit files
(HowTos)

I have filled out the shared folder options in Settings -> Shared Folders of a shared folder with a name of 'share'. It is set to automount but when I start up the guest OS, nothing is showing up in the /media folder.